This invention relates to a two-way automatic communications system (TWACS®) used by electrical utilities for communicating over power transmission lines; and, more particularly, to an inductor based signaling circuit used in generating TWACS signals.
In two-way communications involving TWACS, transponders are employed. At a user site, the transponder is installed in the facility's electric meter. Messages (commonly referred to as “outbound signals”) are sent from a central location (e.g., a sub-station) to the user site to determine if an outage has occurred at the site, inquire about current electrical usage, turn an appliance “on” or “off”, etc. The transponder in the electric meter receives and decodes the outbound signal, and when a response (commonly referred to as an “inbound signal”) is to be transmitted back to the central location, generates and transmits the signal.
In creating signals transmitted via a TWACS, a current pulse of positive or negative polarity is imposed on a half cycle of the AC waveform transmitted over the utility's power lines, this being done using a communications channel involving one or more of the various phases of the AC, and neutral. Formulating an inbound signal requires using different patterns of positive and negative pulses to create the message being transmitted. Some of these pulse patterns have more pulses of one polarity than the other. As part of the message formation, the respective pulses are propagated through the inductor employed by the transponder with positive pulses causing current flow in one direction through the inductor and negative pulses causing current flow in the opposite direction. Over time, the flow of current through the inductor, in one direction or the other, will affect the level of magnetization of the core around which the inductor is wound.
In creating a TWACS inbound communications using an inductor made with a magnetic core material, the current pulse (which resembles a DC pulse) causes the core to be magnetized in one direction. When a pulse occurs that produces current flow in the opposite direction, significant energy is used to overcome this magnetization which results in a pulse having a smaller amplitude than it would otherwise have. During the creation of a TWACS inbound message, multiple pulses of each polarity are produced and a buildup in core magnetization can result. The effect of this causes each inbound pulse to have a variable and substantially unpredictable magnitude. The resulting irregular amplitudes of the signals imposed on the AC waveform so affects the resulting inbound signal as to, in some applications, be outside of the design criteria for the system. One solution to this problem involves modifying or changing the inductor used in the transponder but doing so has size implications which may make it difficult for the transponder to fit within its allotted space in the electric meter; and, there are weight and cost implications to be considered as well.
The present invention provides an effective solution to this problem while negating the need for a new inductor or other system modifications.